Bava Metzia 83b - Nezek

The first mishnah in HaChoveil states that one of the compensatory payments paid by a person who damages another person is Nezek, which is evaluated by calculating the depreciation between how much the victim would have been worth had he been an eved being sold in the market prior to his disability and how much he would be worth now.

Rashi and R' Yehonasan of Lunel explain the rationale for this payment that the damager caused the victim a loss of this amount of money, that if he were to need money, he could have sold himself as an eved ivri.

This interpretation is difficult, because the mishnah which mentions an eved sold in the marketplace is clearly referring to an eved k'na'ani, as the proscription of
לא ימכרו ממכרת עבד (Vayikra 25:42) prohibits selling an eved ivri in a public manner.

The Rosh explains that we view the victim as if he were an eved k'na'ani being that an eved ivri has no method through which he could be sold for perpetuity, because the closest equivalent would be for him to sell himself for six years and six years and six more years for the rest of his life (going free whenever yoveil comes and starting the cycle again).

The Pilpula Charifta (aka the ba'al Tosfos Yom Tov) explains that the mishnah prescribes using the calculation based on an eved k'na'ani rather than the modified eved ivri method that is rejected by the Rosh for two reasons. Firstly, the modified eved ivri method is not used in normal business transactions, so that it's preferable to utilize a method that finds application in the real world to evaluate the victim's value (and going a step further outside, the modified eved ivri method would require an actuary to calculate expected depreciation, fluctuations in the slave market, and life expectancy, thereby making it overly complicated) and secondly, evaluating the victim based on individual six-year sales would be overly harsh on the damager, as throughout nezikin, evaluations are made based on wholesale quantities, rather than on smaller quantities, and using a set of six-year sales would inflate his value over what it would be for a single sale of an equivalent duration.